1. Field of the Disclosure
The present disclosure relates generally to an apparatus and a method for the measurement of the visual appearance of randomly and regularly (i.e., non-randomly) arranged birefringent fibers, such as for example textile fibers and hair.
2. Background Art
Polarization imaging is being used in many applications. In passive imaging, where the illumination is not controlled, i.e., not actively polarized, polarization imaging can be used, for example, to enhance the contrast between human made objects that have a strong polarization signature from the natural background. It also provides information on the shape of the objects. Further, polarization imaging conveniently allows for the detection of water or mud surfaces thanks to the strong polarization signature of water.
In active imaging, where the illumination is controlled and polarized, polarization imaging can be used to study light scattering. In fact, two-state polarization allows for easy separation of the surface scattering from the volume scattering. Specular reflections and color information, which determine the visual appearance of an object, can therefore be separated, thus providing information on the structure of the studied object.
Polarization imaging is applied, for example, in the cosmetic industry with the aim of studying the visual appearance (of the skin, the hair, etc.). Polarization imaging can be used as a tool to improve formulation for both hair and skin care products, such as styling products, for example by visualizing the improvement of the structure and appearance of hair once the product is applied.
The light reflected by birefringent fibers, such as human hair, contains components from the different interactions of the light with the fibers. The three components that may be observed are the following:                light coming from the external reflection, i.e., light that is reflected on the external surface of the fiber. The externally reflected light has the same wavelength (color) as the incident light.        light coming from the internal reflection on an internal surface of the fiber. Since this component propagates through the fiber, it experiences a change of wavelength.        diffused light from volume scattering inside the fiber.This general situation is depicted in FIG. 1 with a single fiber 3a. The visual appearance of the fibers is based on these three different contributions.        
Currently, no system or method allows for physically separating these three components in randomly and regularly arranged birefringent fibers. A method is known for separating the specular reflection component, that contains information on the internal reflection (color) and external reflection (shine) components, from the diffused light using polarization imaging (in: Journal of Cosmetic Science, Bossa Nova Tech, 60, 153-169, March-April 2009) in regularly arranged fibers. Two images corresponding to two couples of polarization states are acquired, a couple of polarization states corresponding to the polarization of the illumination and the observation channel. Using an algorithm, the color and shine signals of the specular light are analyzed so as to separate the internal reflection component from the external reflection component as functions of the angle of incidence of the illuminating light. However, this method requires that the fibers all have the same orientation because information extraction per pixel is not possible. Further, this mathematical method necessitates making assumptions about the internal reflection.
Therefore, there is a need to provide an improved method and an improved apparatus for the visual appearance measurement of randomly and regularly arranged birefringent fibers by a physical decomposition of the measured light in each image pixel into the internally and externally reflected light and the diffusion components, without the necessity to make any assumptions.